We diagnosed phakomatosis pigmentovascularis type IIIb in an 11-month-old baby who had a giant nevus spilus, a nevus flammeus, and moyamoya disease. Development of the patient was normal until 6 months of age when he developed a sudden onset of focal seizures and left hemiparesis. This patient represents the sixth case of phakomatosis pigmentovascularis type IIIb, including three cases in the Japanese literature, reported thus far. However, to our knowledge, this is the first case with an association to moyamoya disease.